This present invention relates to the testing of semiconductor devices. More specifically, the present invention provides methods and apparatus for creating a temporary electrical connection with a semiconductor device for the purpose of testing the functionality thereof.
The common practice of semiconductor manufacturers is to 100% test their products prior to shipping them to their customers. This testing is performed both at the wafer level, i.e., "wafer sort", where the semiconductors are still in the form in which they were manufactured, and at the package level, i.e., "package sort", after the wafer has been sawn up and the individual chips have been mounted into their protective carriers. To perform these tests, a temporary, non-destructive electrical connection is formed between the semiconductor device and the testing apparatus. The device used to perform this function is generically known as a "contactor".
Individuals with ordinary skill in the art will be familiar with the various types of wafer probe cards and packaged device contactors employed for this purpose. One class of contacting devices is based upon the use of a deformable plastic membrane. A contactor based upon this general concept was disclosed in U.S. Pat. No. 3,596,228, the entire specification of which is incorporated herein by reference for all purposes. This class of contactor holds the promise of a device which is relatively easy to manufacture, and a variety of technologies have emerged which reference this pioneering conceptual work.
On the other hand, the conventional non-membrane style contactor requires a great deal of precision handwork to assemble, requiring expensive, highly skilled direct labor in the creation of the assembly. For example, contactors which employ individual probe needles require precise mechanical parts and skilled assembly labor.
Inherent in the concept of the membrane contactor is that it is to be manufactured using batch-process methods. Conceptually this approach offers the hope of relatively low cost, uniformity, and accuracy in its physical structure. Unfortunately, in practice many companies have found membrane contactors expensive to manufacture in the relatively small quantities (when compared to consumer goods) required by the semiconductor industry.
Another problem with current membrane technologies arises from the way that the membrane is typically utilized. That is, according to the conventional solution, the contact points which interact with the semiconductor are firmly affixed to the flexible membrane. The disadvantage of such a structure will become evident with reference to the following discussion.
The functional concept underlying the use of the flexible membrane is to permit the contacts to move relative to each other in the Z-axis and to thereby accommodate the variability in the heights of the contact points on the device to be tested. The membrane also provides a substrate upon with the contact features are manufactured so that the relative X-Y alignment of the individual contact features is maintained. Finally, the membrane provides a substrate upon which connections between the contact points and the supporting Power Wiring Board ("PWB") can be formed.
In the ideal case, the contact points would be completely free-floating in the Z-axis, as the height of contact points on many real-world semiconductors and semiconductor packages are highly variable. When the contact points on semiconductors were relatively far apart, i.e., when the flexible membrane concept was first introduced, there was sufficient contact-point-to-contact-point Z-compliance to allow for real-world variations from ideal co-planarity.
However, as the semiconductor industry has reduced the size of semiconductor devices and simultaneously increased the number of contact points on a given semiconductor chip, the available distance or pitch between the contact points has been correspondingly reduced. Unfortunately, the inherent compliance of the membrane has not been sufficient to deal with this new challenge. An additional source of difficulty has arisen with the advent of Ball Grid Array (BGA) device packages. The BGA concept introduces contact point height variations which are orders of magnitude worse than those encountered when contacting semiconductors directly. That is, a height difference of 8 thousandths of an inch is tolerated where the typical ball height is only 30 thousandths of an inch! This is more than a 25% allowable variation!
Thus, even though conventional membrane contactors have historically been appropriate for a variety of grid array devices, e.g., BGA, land grid array (LGA), and Flip-Chip or Direct Chip Attach (DCA), the distance between contact points on state of the art devices has been reduced to the point at which sufficient Z-compliance is not possible for the conventional designs. Moreover, conventional independent suspension solutions such as, for example, pogo pins, are also decreasing in efficacy. Not only are they considerably more complex and expensive to manufacture than membrane style contactors, as contact pitches decrease the difficulty and expense of manufacturing them increases dramatically.
It is therefore desirable to provide a membrane contactor design which utilizes the advantages inherent in membrane contactors while providing an improved Z-compliance for the contact features which is appropriate to the variability and density of current devices.